The present invention relates to an improved transfer system for transferring a developed electrostatic image from a xerographic plate to a transfer member such as a sheet of paper. More particularly, the invention is directed to a transfer system including an electrically biased transfer roll and a transfer corona generating device.
In conventional xerography, a xerographic plate comprising a layer of photosensitive insulating material affixed to a conductive backing is used to support electrostatic latent images. In the xerographic process, the photosensitive surface is electrostatically charged, and the charged surface is then exposed to a light pattern of the image being reproduced to thereby discharge the surface in the areas where light strikes the surface. The undischarged areas of the surface thus form an electrostatic charge pattern (an electrostatic latent image) conforming to the original pattern. The latent image is then developed by contacting it with a finely divided electrostatically attractable powder referred to as "toner." Toner is held on the image areas by the electrostatic charge on the surface. Where the charge is greater, a greater amount of toner is deposited. Thus, a toner image is produced in conformity with a light image of the copy being reproduced. Generally, the developed image is then transferred to a suitable transfer member (e.g., paper), and the image is affixed thereto to form a permanent record of the original document.
In the practice of xerography, the transfer member is caused to move in synchronized contact with the photosensitive surface during the transfer operation, and an electrical potential opposite from the polarity of the toner is applied to the side of the paper remote from the photosensitive surface to electrostatically attract the toner image from the surface to the paper.
A modern high speed duplicating machine sometimes utilizes an electrically biased transfer roll to effect the image transfer. Although a biased transfer roll system effects very good to excellent copy quality, certain copy quality defects are still produced. These copy quality deficiencies may be classified into two general catagories, poor transfer of images (hollow characters and blotchy solids), and the redistribution of toner in and around images (toner disturbances, Lichtenburg figures, and blur). These deficiencies are present to some extend in other prior art transfer systems.